The present invention relates to digital systems, and more particularly, to such systems for the transmission or recording of color television component signals.
To record component video signals, the component signals are digitized (sampled and then quantized). The quantized signals are most typically represented using 8-bits each (256 grey levels). The digital bits are then recorded using, e.g. a magnetic recording head proximate a moving magnetic tape. To reproduce the signal, the tape is displaced past a magnetic reproducing head thereby inducing the originally recorded digital signals therein. The digital signals then are converted to analog component signals. If desired the component signals may then be combined to form a composite video signal.
One possible choice for the component signals is R, G, and B (red, green, and blue respectively). However each of the recording channels (one channel for each of the component signals) must then have the full bandwidth of the video signal, e.g. 4.2 MHz for NTSC, in order to achieve a high resolution picture upon reproduction. It is possible to use the principle of "mixed highs" to reduce the required bandwidth in an R, G, B system such as is disclosed in the paper entitled "Primary Signal Component Encoding" by I. G. Brown presented at the 1980 International Broadcasting Convention, conference publication No. 191. Another possibility is to record Y(luminance) and color difference signals, such as R-Y and B-Y or I (inphase) and Q (quadrature) in the three channels respectively. This choice of recorded signals permits using a wideband, e.g. 4.2 MHz, channel for only the Y signal and narrower bandwidth channels for the color difference signals, and therefore is an improvement in this respect over recording the R,G, and B signals but is the same as mixed highs in this respect. However, the dynamic range required in this case is increased over that required for an R,G, and B system.
This is illustrated in FIG. 1, where the horizontal axis represents the Y signal and the vertical axis represents the R-Y color difference signal. To construct the graph the equation Y=0.1B+0.3R+0.6G is used, which is a reasonably accurate approximation and further, it is assumed that the R,G, and B signals each have a normalized dynamic range of 0 to 1. The signal R-Y is equal to R-(0.1B+0.3R+0.6G)=-0.1B+0.7R-0.6G. When R=B=G=0, both signals R-Y and Y are 0, which is represented by origin point 0 in FIG. 1. When R=1 and B=G=0, then R-Y=0.1(0)+0.7(1)-0.6(0)=0.7 and Y=0.1(0)+0.3(1)+0.6(0)=0.3 which is represented by point A. When R=B=G=1, then Y=0.1( 1)+0.3(1)+0.6(1)=1 and R-Y=-0.1(1)+0.7(1)-0.6(1)=0, which is represented by point B. When R=0 and B=G=1, then R-Y=-0.1(1)+0.7(0)-0.6(1)=-0.7 and Y=0.1(1)+0.3(0)+0.6(1)=+0.7, which is represented by point C.
The interior area bounded by parallelogram OABC represents all possible combinations of the signals Y and R-Y. It is noted that the required dynamic range for the R-Y signal corresponds to the range of points A to C, which equals +0.7-(-0.7)=+1.4 relative to that required for recording R,G and B signals.
A similar diamond shaped figure results when the B-Y signal is graphed versus the Y signal although the exact numbers are different. For example, the signal B-Y=1B-(0.1B+0.3R+0.6G)=0.9B-0.3R-0.6G. If at one extreme of the B-Y signal B=1 and R=G=0, then the B-Y signal equals 0.9, while, if at the other extreme B=0 and R=G=1, then B-Y=-0.3-0.6=-0.9. Thus the required dynamic range for the B-Y signal is +0.0(-0.9)=1.8.
Thus the recording of the Y, B-Y, and R-Y signals requires a larger dynamic range in the color difference channels than recording R,G, and B signals where all channels have a dynamic range of one. A larger dynamic range has the disadvantage in a digital system that more quantizing levels are required for a given quantizing error.
Another problem that occurs in either recording or transmission of signals is that of dropouts, which are a temporary loss of signal information. To overcome this problem, it is customary to separate the information signal into a plurality of channels, the channels being in the case of a tape recording system either a plurality of independent recording tracks or a single track having all channels, but with a time delay therebetween longer than the expected dropout length, such as shown in U.S. patent application Ser. No. 241,925 filed Mar. 9, 1981 in the name of Glenn A. Reitmeier and assigned to the assignee of the present invention. A dropout will most probably affect only one channel, thus allowing the dropout to be concealed using the information in the remaining channels. However, in said application half the pixels (picture elements) are represented in each channel, and therefore the concealment process results in one half the resolution of the original picture signal for the duration of the concealment.
It is therefore desirable to have a signal format that provides a dynamic range that fits the transmission channel and has high dropout concealment capability.